A variable interest rate loan is a loan in which the interest rate charged on the outstanding balance varies as market interest rates change. As a result, your payments will vary as well (as long as your payments are blended with principal and interest).
Fixed interest rate loans are loans in which the interest rate charged on the loan will remain fixed for that loan's entire term, no matter what market interest rates do. This will result in your payments being the same over the entire term. Whether a fixed-rate loan is better for you will depend on the interest rate environment when the loan is taken out and on the duration of the loan.
When a loan is fixed for its entire term, it will be fixed at the then prevailing market interest rate, plus or minus a spread that is unique to the borrower. Generally speaking, if interest rates are relatively low, but are about to increase, then it will be better to lock in your loan at that fixed rate. Depending on the terms of your agreement, your interest rate on the new loan will remain fixed, even if interest rates climb to higher levels. On the other hand, if interest rates are on the decline, then it would be better to have a variable rate loan. As interest rates fall, so will the interest rate on your loan.
This discussion is simplistic, but the explanation will not change in a more complicated situation. It is important to note that studies have found that over time, the borrower is likely to pay less interest overall with a variable rate loan versus a fixed rate loan. However, the borrower must consider the amortization period of a loan. The longer the amortization period of a loan, the greater the impact a change in interest rates will have on your payments.
Therefore, adjustable-rate mortgages are beneficial for a borrower in a decreasing interest rate environment, but when interest rates rise, then mortgage payments will rise sharply. Use a tool like Investopedia's mortgage calculator to estimate how your total mortgage payments can differ dependent on which mortgage type you choose.